This invention relates to an aqueous coating composition and in particular to an aqueous acrylic coating composition or enamel for electrical conductors.
Conventional aqueous acrylic coating compositions or enamels are widely used to coat wires and other metallic conductors and in general form high quality coatings having good electrical properties. Examples of these enamels and processes for making them are shown in:
Sanders U.S. Pat. No. 2,787,561 issued Apr. 2, 1957; Sanders U.S. Pat. No. 2,787,603 issued Apr. 2, 1957; Sanders U.S. Pat. No. 2,866,763 issued Dec. 30, 1958; Sanderson U.S. Pat. No. 3,032,521 issued May 1, 1962; Bullitt U.S. Pat. No. 3,069,375 issued Dec. 18, 1962; Fitch U.S. Pat. No. 3,104,231 issued Sept. 17, 1963; Victorius U.S. Pat. No. 3,206,421 issued Sept. 14, 1965 and Mesec U.S. Pat. No. 3,386,939 issued June 4, 1968.
A particular problem that occurs with these conventional enamels when used as coatings on wire for medium power transformers and distribution transformers is that the coatings have relatively poor high temperature dielectric strength and lose flexibility on aging. Surges of high voltages of electricity, for example, caused by lightning striking a transformer, cause a breakdown of these enamel coatings. Lightning striking a transformer causes wire used in the transformer to vibrate severely and if the coating on the wire is embrittled, the coating will shatter. Also, if the coatings' dielectric strength is insufficient, a breakdown will occur. There is a need for an enamel that has an improved dielectric strength particularly at elevated temperatures and will not embrittle on aging.